package Beginners;

import java.io.PrintWriter;
import java.util.Scanner;

class Solver1068
{	int n,rs;
	
	public void Input()
	{	Scanner in=new Scanner(System.in);
		n=in.nextInt();
		in.close();
	}
	public void Solve()
	{	if (n==0)
			rs=1;
		else if (n>=1)
			rs=(n*(n+1))/2;
		else
		{	int k=-n,tmp=(k*(k+1))/2;
			rs=-tmp+1;
		}
	}
	public void Output()
	{	PrintWriter out=new PrintWriter(System.out);
		out.println(rs);
		out.flush();
	}
	public void main()
	{	Input();
		Solve();
		Output();
	}
}
public class P1068
{	public static void main(String[] args)
	{	Solver1068 solver=new Solver1068();
		solver.main();
	}
}
